Along with rapid development toward portable, smaller size, lower weight, higher performance electronic devices in the electronic and information communication industries, there is an increasing demand for high-capacity and high-performance lithium secondary batteries as a power source for these electronic devices. Furthermore, along with commercialization of electric vehicles (EVs) or hybrid electric vehicles (HEVs), research into lithium secondary batteries having high capacity, high power output, and high stability has been vigorously conducted.
A lithium secondary battery may be manufactured by forming a negative electrode and a positive electrode using materials that allow intercalation and deintercalation of lithium ions and injecting an organic electrolyte solution or a polymer electrolyte solution between the negative electrode and the positive electrode. The lithium secondary battery may electric energy through oxidation and reduction reactions that take place as the intercalation and deintercalation of lithium ions occur in the negative and positive electrodes.
Of the ingredients of a lithium secondary battery, a material of the positive electrode is crucial in term of capacity and performance of the battery.
As the first commercialized positive electrode material, lithium cobalt oxide (LiCoO2) has been mostly used till now due to good structural stability and ease of mass production, compared to other lithium transition metal oxides. However, this positive electrode material is expensive due to the limitation of cobalt as a natural resource, and is harmful to the human body.
For these reasons, positive electrode materials as alternatives to such lithium cobalt oxides have been studied in various aspects. In particular, a nickel (Ni)-rich positive active material, i.e., LiNi1−xMxO2 (wherein 0≤x≤0.5 and M may be, for example, Co or Mn), among other lithium metal oxides having a layered structure, may implement a high capacity of about 200 mAh/g or greater, and thus is considered a suitable positive electrode material for next-generation electric vehicles and power storages. Such Ni-rich positive active materials are less toxic to the human body and cost low, and thus have been studied with great interest.
However, Ni-rich positive active materials may cause swelling due to an increased surface residual lithium and generate gas through reaction with electrolyte solution.
For example, a general method of preparing a lithium metal oxide may include preparing a transition metal precursor, mixing the transition metal precursor with a lithium compound, and then calcining a resulting mixture. As the lithium compound, LiOH and/or Li2CO3 may be used. In order to facilitate crystalline structure formation, the thermal treatment is performed with an excess of the lithium-containing compound added, so that a large amount of the residual lithium as LiOH or Li2CO3 may remain unreacted on the surface of the positive active material. Such a residual lithium, i.e., the unreacted LiOH or Li2CO3 may cause gasification and swelling through reaction with an electrolyte solution in the battery, leading to a severe reduction in high-temperature stability.
Patent document 1 discloses a method of suppressing a side reaction between a Ni-rich positive active material and an electrolyte solution by uniformly coating a silicon oxide on the surface of the Ni-rich positive active material. According to Patent document 1, a method of preparing a positive active material may include: preparing a coating solution including a silicon oxide; adding a Ni-rich lithium metal composite oxide having a Ni content of 50% or greater to the coating solution and stirring a resulting solution to coat the silicon oxide on a surface of the lithium metal composite oxide; and thermally treating the lithium metal composite oxide including the silicon oxide coated on the surface thereof at a temperature of about 400° C. to 600° C.
Non-patent document 1 discloses a method of coating a Ni-rich positive active material (LiNi0.6Co0.2Mn0.2O2) with a metal oxide (Al2O3 or TiO2) or a metal fluoride (AlF3) by impregnation and calcining at a temperature of about 450° C.
However, the above-described methods use a wet coating process with a solvent in a positive active material coating process, and thus require an additional drying process after the coating, wherein partially non-uniform coating may occur depending on the conditions of stirring in the drying process, leading to reduced performance improvement. In addition, such simply mixing the coating material and the positive active material may lower coating uniformity, and may also reduce electric conductivity due to the heat treatment at a low temperature after the mixing. Furthermore, the coating material remaining exposed on the surface of the positive active material may lower high-rate characteristics of a battery.